The present invention relates to a potentially nonlinear resistor composed of a sintered product which chiefly consists of zinc oxide, and to a process for producing the same.
In recent years, sintered products obtained by molding and calcining zinc oxide as a chief component, and bismuth oxide, manganese oxide, cobalt oxide, antimony oxide, and, as required, nickel oxide, chromium oxide, silicon oxide, boron oxide, lead oxide, magnesium oxide, aluminum oxide, and the like, as well as sintered products obtained by molding and calcining zinc oxide as a chief component, and lanthanum oxide, praseodymium oxide, samarium oxide, neodymium oxide, or cobalt oxide, manganese oxide, and the like, have been widely used as potentialy non-linear resistors in such fields as voltage stabilizer elements, surge absorbers, arresters and the like.
When the potentially non-linear resistor is used as a high-voltage surge absorber or arrester, the side surfaces thereof are usually covered with a glass layer in order to prevent the creeping flashover.
An arrester of this type has been disclosed, for example, in Japanese Patent Publication No. 26710/79. According to the potentially non-linear resistor disclosed in Japanese Patent Publication No. 26710/79, the glass layer for coating (1) must have strength against the heat cycle, (2) must have resistance against the humidity, and (3) must be easily handled. Therefore, a lead borosilicate glass having a coefficient of thermal expansion of 60 to 85.times.10.sup.-7 /C., or a zinc borosilicate glass having nearly the same coefficient of thermal expansion, or such glasses blended with titanium oxide, aluminum oxide or copper oxide, having been employed. Further, to cover the side surfaces of the resistor with the glass, the glass powder is blended with an organic binder to prepare a glass paste, the glass paste is adhered onto the side surfaces of the resistor and is heated at a temperature of about 400.degree. to 650.degree. C. in an oxidative atmosphere, such that the glass layer is baked.
With the resistor covered with the glass by such a conventional method, however, increased leakage current flows in a low-voltage region as compared with the resistors which are not coated with glass. Namely, the resistor coated with the glass according to the conventional method exhibits poor non-linear characteristics. Referring, for example, to a potentially non-linear resistor having a diameter of 50 mm and a thickness of 22 mm, the non-linearity coefficient .alpha. was 50 in a low-current region of 10 .mu.A to 1 mA (current density of from 4.times.10.sup.-7 to 4.times.10.sup.-5 A/cm.sup.2) before the resistor was coated with the glass. After the resistor was coated with the glass, however, the non-linearity coefficient .alpha. decreased to 20 or less. In practice, however, the potentially non-linear resistor must have the non-linearity coefficient .alpha. of greater than 30. For example, when applied to the arresters for protecting 1,200,000-volt transmission lines, the non-linearity coefficient .alpha. which is smaller than 30 permits a leakage current of greater than 80 .mu.A to flow under a normal voltage ratio (normal operation voltage/voltage when a current of 1 mA is allowed to flow) of 95%. Namely, long life of 100 to 150 years required for the arresters cannot be expected.